The transmission of digital data in both public and private networks relies upon the use of data modems. Modems are modulators/demodulators designed for transmission of incoming data streams over analog facilities such as a conventional voice frequency (vf) (300-3400 Hz) telephone channel. Modems were devised because it was more economical to use the existing analog telephone network rather than constructing new separate digital transmission networks.
A voice frequency telephone connection through the public switched telephone network contains two-wire segments, known as subscriber loops, which connect the sending and receiving stations to their respective end offices. Connection between end offices are made via the network's trunk lines, which may transport signals using analog baseband, analog modulated carrier, digital carrier, or other technologies.
While the loop carries both directions of communication on one pair of wires, the trunks connecting the end offices typically send each transmission direction over a separate pair of wires. This is known as four-wire transmission.
Two-wire loops are connected to the four-wire transmission circuits at the end office through devices called hybrids. A hybrid is a four-port device used to separate signals travelling in both directions along a single pair of wires into individual directions, and to recombine those signals travelling on four-wire circuits for use on two-wire loops.
A conventional telephone hybrid has transmit and receive paths connected to two of the ports, the loop to the third port, and a so-called balance network connected to the fourth port. If the impedance of the balance network is equal to the input impedance of the loop over the frequency range of the signals, then the transmit and receive ports are conjugate ports, that is, these ports are decoupled, and a signal transmitted into the loop will not appear at the modem's own receiver. A measure of how well the hybrid isolates the transmit and receive ports is the transhybrid loss. An ideal hybrid would have an infinite transhybrid loss.
However, in practice, the balance network does not equal the input impedance because of the numerous make-ups in the loop plant. These loops vary because of cable gauge, length, termination, and operating conditions such as temperature. Since these variations are not predictable on a loop-by-loop basis, the usual solution is to select a compromise balance network that, on average, will minimize the "leakage" across the transmit (T) and receive (R) ports or maximize the trans-hybrid loss.
The choice of a compromise balance network depends on the balance technique for the end office. Standard balance is 900 ohms resistance in series with 2.16 .mu.F capacitance. If the loop plant at the end office is segregated into loaded and non-loaded loops, separate networks can be used. For loaded loops, a compromise balance circuit contains a branch containing 100 ohms resistance in series with 5000 pF capacitance in parallel with a branch containing 1650 ohms resistance. Non-loaded loops require a compromise balance circuit of a branch containing 100 ohms resistance in series with 0.05 .mu.F capacitance in parallel with a branch containing 800 ohms resistance.
When a transmission line, such as the two-wire subscriber loop, with characteristic impedance Z.sub.0, is terminated in an impedance Z.sub.t, such as the input impedance of the end office hybrid, which is other than Z.sub.0, part of the signal incident to that termination will be reflected back to the driving point. This phenomena is known as end office reflected echo. It has a profound effect on data transmission in telephone networks.
A voice-band data modem generally has separate internal transmit and receive paths which are combined to effect communication over the two-wire loops. A data source in a near-end modem generates data signals for transmission to a far-end modem, whereas an independent data source in the far-end modem propagates data signals which serve as the receive data signals for the near-end modem. Generally a hybrid is used to couple the independent transmit and receive sections of a modem to the loop.
Leakage from the T port to the R port due to the mismatch between the modem's balance network and the loop input impedance leads to a so-called network interface echo condition in a modem wherein an attenuated portion of the transmit data signal interferes with the incoming received data signal.
Both the network interface echo and the end office echo cause errors in the detected data signal; in general, the larger the echo, the greater the error rate. The severity of both of these echoes is a function of the degree of mismatch between the modem's balance network (Z.sub.bm) and the input impedance of the loop (Z.sub.IN). To test modem performance in a laboratory setting, especially the response of a modem to an echo condition, a testing arrangement is required whereby various mismatch conditions representative of those which occur in an actual field setting may be conveniently simulated.
Such a requirement basically translates into the ability to simulate the input impedance or driving point impedance of various loop make-ups. Conventionally, so-called artificial cable elements have been devised that facilitate simulation of numerous loop make-ups. The components of the kit are lumped element representations of various gauges and lengths of distributed transmission lines. Each component provides a reasonable estimate to the behavior of the particular gauge and length of line being simulated over the voice frequency range. Different make-ups are simulated by plugging together the components having fixed lengths to achieve the desired length. However, cable kit elements merely serve to replace a distributed line in the sense that the kit has input and output ports which replicate the input and output ports of the line.
The major drawback associated with the artificial cable method of simulation of input impedance is the fact that it introduces effects due to cable parameters other than the impedance. What is needed is a method of impedance synthesis that is able to fully exercise the modem circuitry, is free of unwanted effects, and allows the conditions of the test to be fully controlled.